The invention relates to an air compressor with a stepped piston movably disposed in a cylinder and which performs compression in both stroke directions, whereby each compression stage is associated with at least one inlet and one outlet valve, and whereby an outlet opening of a lower compression stage is connected with an inlet opening of a higher compression stage.
Air compressors with two compression stages are used to produce compressed air for the brake system in trucks. The compressor is flanged to the vehicle motor and is driven continuously. Because of the continuous drive, a pressure regulator opens a suction valve of the first stage when the intended pressure is reached. The dead volume of the second stage must be such that the compressor "runs dead" under the lowest operating pressure of the system. It is also possible to arrange a pressure regulator in the supply line of the compressor, whereby the regulator opens the supply line to the atmosphere when a prescribed high pressure is reached.
The provision of compression in both stroke directions assures smooth running and a compact construction. The latter is particularly important because of the limited available space in the engine compartment of a truck.
The use of this type of stepped piston compressor to produce compressed air for the brake system and for starting diesel truck engines is described in the magazine "Diesel And Gas Turbine Progress" of September, 1969.
The stages of a stepped piston compressor are necessarily disposed in different planes of the cylinder. In known two-stage compressors the valves of the first stage are disposed above or at the side of the upper end of the cylinder in the vicinity of the cylinder head. This enables the inlet and outlet valves to be designed independently of each other or, in other embodiments, to be combined with each other, i.e. the same valve can be provided with inlet and outlet functions. The separate inlet and outlet valves of the second stage are disposed in the lower portion of the cylinder.
These known compressors exhibit various disadvantages. They have a great number of individual parts and are therefore expensive. The different locations of the valves makes mounting difficult. Depending on the placement of the compressor on the vehicle motor, the side inlet and outlet valves of the second stage are either inaccessible or only accessible with a great deal of difficulty. To correct or check them the entire compressor must be removed. Maintenance and repair costs are therefore very high.